The present invention concerns a thread-cutting mechanism for a sewing machine, especially an eyelet-buttonhole sewing machine with a moving blade and a stationary blade. The moving blade travels back and forth in a groove in a plate with an aperture for the needle to pass through while it is producing a stitch.
German B 1 104 805 discloses a thread-cutting mechanism of this genus in a double saddle-stitch sewing machine. A blade is positioned above a slot below the groove in such a plate and secured to a web thereon. The edge of the blade extends into the path traveled by a cutting edge on the bottom of a slide that travels back and forth in the groove. An extension on the side of the blade is bent down and rests on a web that is part of the plate. The slide itself is attached to an actuating slide. To cut the threads, the actuating slide is extracted manually, whereupon the cutting edge on the bottom of the first slide intercepts the threads and draws them tight over the upper blade, severing them. Springs attached to the actuating slide return it to its original position when released. The underthread and the needle-thread loop are severed while the two last stitching points on the material being sewn are equidistant from the needle aperture. The tails that remain with the material are accordingly approximately equal in length, which depends on the particular stitch length.
U.S. Pat. No. 2,752,871 discloses a thread-cutting mechanism on an eyelet-buttonhole sewing machine with a moving blade accommodated below the needle-aperture plate but with no stationary blade. The moving blade is bent into an L and secured to an aligning bracket. The aligning bracket itself is accommodated in a holder. The blade's alignment point is accordingly far from its cutting point. Blades of this type cannot be made of already tempered sheet steel. Such bent blades again cannot be ground to ensure dimensional stability. These thread-cutting mechanisms are accordingly not entirely reliable. Since the mechanism operates without a stationary blade to sever the underthread, it is difficult to cut thicker threads, threads with a count higher than approximately 50/3 for example. Since the needle-aperture plate and the thread-cutting mechanism are screwed tight independently, the rest of the underthread dictates the precise adjustment of the needle aperture in relation to the cutting mechanism.
The thread tails are left free subsequent to its being severed in the aforesaid thread-cutting mechanisms. The uncontrolled situation is detrimental to reliable re-initiation of the seaming procedure (stitch forming).
The buttonhole is opened at one point and sewn at another in the eyelet-buttonhole sewing machine distributed under the designation Durkopp Adler Kl. 558 by the present applicant. Once the buttonhole has been sewn, the material is advanced to a blade mounted stationary on the machine for cutting. Subsequent to cutting, the looper base is returned to its initial position, whereby the motion that actuates the looper-thread blade rotation derives from the rotation of the looper base. Since the blade is stationary, the buttonhole can be opened only at a specific point. The eyelet is accordingly always the same distance from the sewing point (needle) no matter how long the buttonhole is, whereas the position of the corner of the buttonhole varies with that length. The trimmed looper-thread tails remaining in the material accordingly depend on the length of the buttonhole. The end of a short buttonhole will be farther from the sewing point than the end of a long buttonhole. The length of the trimmed tails will consequently vary inversely with the length of the buttonhole. The needle thread is severed from the looper thread with no additional blade.